Dansylhydrazine reacted with aldehyde groups in collagen. The densyl fluorescence of aged collagen exhibited a weaker peak at 525nm, whereas that of young collagen had a stronger, broad peak at 500nm. Fibril formation in vitro was partially inhibited. During the turbidity lag phase, the dansyl fluorescence increased (30-50%). These changes reveal the telopeptide conformation changes occurring during this period. Upon UV(340nm)-irradiation, fibril formation was almost completely inhibited. With increasing temperature, the densyl fluorescence of young collagen decreased, whereas that of old collagen substantially increased. After denaturation, both fluorescences became similar in their intensity and position (490nm). The fluorescence (545nm) of turbium ions (Tb3ion) was enhanced by energy transfer from tyrosine to Tb3 ion bound to collagen and increased slightly during the turbidity lag phase. Tb3 ion also increased the rate of fibril formation, suggesting stronger binding sites on polymeric collagen. Hyaluronic acid (HA) from rooster comb, after chloroform treatment to remove proteins, exhibited no detectable UV absorption, but was degraded by UV (254nm) light, suggesting a highly efficient photo-degradation. The UV (255nm) peak persisted throughout deproteinization for HA from umbilical cord. This HA was also photo-degraded. UV-absorbing species are tissue-dependent and may not be correlated with proteins.